The title alone is grounds for an uprising, but bear with me. It’s necessary for certain OS X users to run Windows off their Mac’s. Since Apple made the switch to Intel, Bootcamp has come standard for dual booting Windows or just about any other OS for that matter.
While dual booting is just fine for many users, virtualizing Windows inside of OS X is much more efficient than rebooting for a few routine tasks. Parallels 
seems to be your best option for this task.
One of the coolest features, Coherance, allows you to run Windows programs as individual windows instead of virtualizing the entire desktop. This makes it super easy to do things like drag and drop between OS X and Windows applications. Not to mention you can also use Exposé with your virtualized windows.
Coherance is demoed below by our friends at LifeHacker:
You can also open Windows files with OS X and OS X files with Windows. You can even map OS X drives to Windows letter drives for easy access to all your Mac files.
Alternatively, there’s VMware Fusion which has fewer features and runs a little slower than Paralles in most scenarios.
MacTech recently did a great benchmark on Parallels, Bootcamp and Fusion. Each application performs certain tasks better depending on your needs.
Summary of Options
Bootcamp: If you don’t need virtualization–Bootcamp is your best option, hands down. Price: Free!
Parallels : If you want to virtualize Windows XP, Parallels is slightly faster and more user friendly. Price: $79 to $99
Fusion: If you’re interested in running Windows Vista, Fusion performs slightly better. Price: $79
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Omer Zach
Brad Jasper
MacPhilly aka Philllip Zannini
I started with Parallels after reading several of the evaluations, but in general found that running the single window mode slowed my OS X interaction far too much for me want to use it - and found I couldn't run in coherence mode without dragging my machine to a stop.
In frustration, I downloaded and installed Fusion one Saturday. OH MY WORD. I couldn't believe the difference in performance. Fusion runs with OS X seamlessly, INCLUDING Unity mode (the equivalent of Coherence in Parallels). My Windows VM booted faster. My OS X interaction didn't slow at all and all my applications (like Microsoft Visio) worked great in both the single window and Unity modes.
Is Parallels bad? Maybe not if you have a big honking machine, but for my money, Fusion with it's configurable processor settings works WAY better and is completely worth the $80. Parallels = 3 1/2 stars. Fusion 4 1/2. December 22nd, 2007 at 10:20 pm
Brad Jasper
I am surprised, however, to hear you say Fusion was faster for you as Parallels did better on the benchmarks.
Are you using Windows XP with Fusion? December 22nd, 2007 at 11:45 pm
Travis
Omer Zach